1. Field of the Invention
The invention relates to a system and method for monitoring cerebral compliance in a patient and more particularly to comparing repeatedly, preferably continuously, signals from at least two pressure sensors placed at different sub-dural locations, preferably sub-meningeal locations, within the brain of the patient to detect changes in cerebral compliance. Alternatively, two or more generated reference frequencies are measured at one or more pressure sensor locations.
2. Description of the Related Art
Human brain tissue includes gray and white matter suspended in cerebrospinal fluid within the cranium and nourished by blood delivered through cerebral arteries. The gray matter has closely spaced cell bodies of neurons, such as in the cerebral cortex, and the underlying white matter contains densely packed axons that transmit signals to other neurons. Brain tissue has different densities and comprises approximately eighty percent of the intracranial content, with blood and cerebrospinal fluid each normally comprising approximately ten percent.
Cerebrospinal fluid is produced in several connected chambers known as ventricles and typically is renewed four to five times per day. Cerebrospinal fluid in a healthy human flows slowly and continuously through the ventricles, propelled by pulsations of the cerebral arteries, flows around the brain tissues and the spinal column, and then through small openings into the arachnoid membrane, which is the middle layer of the meninges surrounding the brain parenchyma and ventricles, where the fluid is finally reabsorbed into the bloodstream.
Under normal conditions, bodily mechanisms compensate for a change in fluid volume within the cranium through tissue resilience and by adjusting the total volume of blood and cerebrospinal fluid so that a small increase in fluid volume does not increase intracranial pressure. Similarly, a healthy brain compensates for an increase in intracranial pressure to minimize a corresponding increase in intracranial volume. This volume- and pressure-relationship can be explained in terms of cerebral compliance, which term is intended to include herein the terms elastance and intracranial compliance.
The brain is compliant as long as a person's auto-regulatory mechanism can compensate for any change in volume. As soon as the brain's auto-regulation or compensatory mechanisms fail, blood and cerebrospinal fluid cannot be displaced, and the brain can no longer adapt to any increase in fluid volume.
A reduction in cerebral compliance eventually will lead to an undesired increase in intracranial pressure, such as described by Seder et al. in “Multimodality Monitoring in Patients with Elevated Intracranial Pressure” from the book “Intensive Care Medicine” published by Springer New York (2008). Reduced cerebral compliance is also referred to as increased brain stiffness or as stiff brain. As more fluid volume is added, a threshold is reached beyond which small increases in volume lead to dramatic and unhealthy increases in intracranial pressure.
Intracranial pressure has been measured at a number of epi-dural and sub-dural locations, such as described by Steiner et al. in “Monitoring the injured brain: ICP and CBF”, British Journal of Anaesthesia 97(1): 26-38 (2006) and by Brean et al. in “Comparison of Intracranial Pressure Measured Simultaneously Within the Brain Parenchyma and Cerebral Ventricles”, Journal of Clinical Monitoring and Computing 20: 411-414 (2006).
In an early method of determining cerebral compliance, one or more volumes of fluid were added intracranially to produce intracranial pressure variations that were studied by directly measuring intracranial pressure. Cerebral compliance has been estimated over the years by various techniques including studying cerebral perfusion pressure, which has been calculated by measuring intracranial pressure and then subtracting it from systemic blood pressure or mean arterial pressure to obtain a cerebral perfusion pressure value such as described by Portella et al. in “Continuous cerebral compliance monitoring in severe head injury: its relationship with intracranial pressure and cerebral perfusion pressure”, Acta Neurochirurgica (Wein) (2005) 147: 707-713. In some procedures, a ventricular catheter has been placed in a brain ventricle to continuously monitor intracranial pressure while an indwelling radial artery catheter with pressure transducer measures mean arterial pressure.
The Spiegelberg system uses a double lumen ventricular catheter having an air pouch mounted at its tip. Cerebral compliance is calculated from a moving average of small increases in intracranial pressure caused by up to several hundred pulses of pouch-added volume. A stable average is developed and then mean cerebral compliance is measured minute-by-minute, which is also described in the above-referenced Portella et al. article. However, this technique can have a poor frequency response, that is, one or more minutes may pass while the Spiegelberg system only posts a single mean value.
In U.S. Publication No. 2009/0143656, Manwaring et al. describe certain systems and methods of measuring intracranial pressure and determining cerebral compliance by detecting phase shifts in pulsatile perfusion flow signals derived from a first noninvasive intracranial flow sensor, such as an oximeter positioned on the forehead next to a supraorbital artery or a tympanic membrane displacement sensor positioned in the ear canal, and a second noninvasive extracranial sensor.
Several techniques for obtaining and processing pressure-related signals are described by Eide in U.S. Pat. Nos. 7,335,162 and 7,559,898 and U.S. Publication No. 2009/0069711 using one or two intracranial pressure sensors, either alone or with an epi-dural or extracranial sensor.
Kucharczyk et al. in U.S. Pat. No. 6,537,232 disclose a device and method for monitoring intracranial pressure during magnetic resonance image-guided procedures such as intracranial drug delivery. One or more pressure sensors are positioned along a catheter to deliver feedback as fluids are injected or withdrawn. Multiple pressure sensors are utilized to detect and measure pressure gradients during drug delivery.
It is therefore desirable to have a simpler, more rapid and more accurate technique for monitoring cerebral compliance.